1. Field of the Invention
The invention relates to radio frequency amplification stages, and especially to the biasing of these stages.
2. Description of the Related Art
Radio frequency amplification stages have specific constraints according to their functions.
Thus, the stages which process weak signals in reception require low consumption, a low noise level, a high compression point, weak small signal intermodulation, and a controlled drifting of the quiescent current with temperature.
Transmission amplification stages on the other hand require process-dependent stabilized consumption, a low noise level, a high compression point, weak large signal intermodulation, and a controlled drifting of the quiescent current with temperature.
It is recalled here that the compression point of an amplifier stage is the input power beyond which the corresponding output power deviates by 1 dB from the theoretical output power corresponding to linear operation of the stage.
Thus, the higher the compression point, the bigger the input power range corresponding to a zone of linear operation.
Moreover, intermodulation is manifested by the appearance of intermodulation lines in a communication channel, causing degradation of the signal/noise ratio, or else a phase trajectory error, and in the particular case of transmission, pollution of the adjacent channels.
This intermodulation may be manifested in reception by the value in dBm of an order 3 interception point (IIP3) according to terminology well known to the person skilled in the art. The higher the value of this point, the weaker the intermodulation and consequently the better the reception.
This intermodulation can be manifested in transmission by the value of the power ratio of the adjacent channel (ACPR: xe2x80x9cAdjacent Channel Power Ratioxe2x80x9d) according to terminology likewise well known to the person skilled in the art.
And the smaller this ratio, the weaker the intermodulation.
It may be shown that most of the performance required for the amplifier stage depends a great deal on the biasing block adopted for this amplifier stage. Currently, design techniques based on discrete components, emanating from AsGa technology, are widely used, for example in biasing circuits of the type of that which may be found in the power amplifier from the company RFMD, referenced RF2138, and used for example in wireless communication systems based on the GSM standard.
However, this type of biasing circuit suffers from two major drawbacks, namely poor control of the current, giving rise to an absolute error and a drift with temperature, and a static and dynamic impedance exhibited on the base of the radio frequency amplifier transistor, which is equivalent to a static resistance. In fact, exhibiting a static resistance on the base of the radio frequency transistor gives rise to a limitation of the compression point, as well as to a degradation of the noise of the amplifier stage and a reduction in the linearity performance of the stage.
More recently, an article by Stephen L. Wong, entitled xe2x80x9cA 2.7-5.5 V, 0.2-1 W BiCMOS RF Driver Amplifier IC with Closed-Loop Power Control and Biasing Functionsxe2x80x9d IEEE, Journal of Solid-State Circuits, Vol. 33, No. 12, December 1998, as well as the corresponding U.S. Pat. No. 5,760,651, have described a biasing circuit based on the active simulation of an inductance and using an operational voltage amplifier to copy the reference voltage of a diode onto the base of the power transistor. This reference diode is connected to an independent ground of the radio frequency circuit, thereby greatly limiting the benefits that may be derived from this type of bias.
Moreover, during use in transmission mode, this configuration also limits the characteristics of the adjacent channel power ratio (ACPR) which can be obtained.
Finally, this type of embodiment is subject to serious dispersions in quiescent current on account, on the one hand, of the offset voltage of the biasing amplifier, and, on the other hand, of the inaccuracy of the ratio of geometry between the reference diode and the radio frequency amplifier transistor.
The article by Sifen Luo, entitled xe2x80x9cA Monolithic SI PCS-CDMA Power Amplifier with an Impedance-Controllable Biasing Schemexe2x80x9d, 2001, IEEE, International Microwave Symposium, describes a more complex biasing scheme, based on current mirrors with two control currents. Thus, one of the currents is presumed to control the quiescent current in the final power stage, whilst a second current separately controls the impedance exhibited by the output of the biasing stage.
Now, the control of the quiescent currents in the transistor of the final power stage remains very inaccurate, both in terms of absolute value and also temperature. Furthermore, the system for controlling the output impedance through the second current substantially influences the final bias current of the power stage, it being impossible to disregard this when it is known that one is dealing with that stage of a mobile telephone having the greatest consumption.
Furthermore, this arrangement does not make it possible to achieve high compression points on account, in particular, of the base resistance which tends to de-bias the transistor when its base current increases.
An article by Eiji Taniguchi, entitled xe2x80x9cDual Bias Feed SiGe HBT Low Noise Linear Amplifierxe2x80x9d 2001, IEEE, International Microwave Symposium, again presents the problem of the limitation of compression inherent in biasing stages with current mirrors possessing a resistance to access to the base of the radio frequency transistor, which is generally of high value for noise considerations.
The solution advocated in this article consists in short-circuiting this biasing stage, through a network of three diodes. However, such a solution suffers from the drawback of exhibiting a point of triggering of the short-circuit transistors which depends on the pairing of the radio frequency transistors and on the chain of these three short-circuit transistors. The reproducibility of performance may therefore be jeopardized. Moreover, this type of solution does not afford any improvement as regards intermodulation.
Finally, an article by Keng Leong Fong, entitled xe2x80x9cHigh-Frequency Analysis of Linearity Improvement Technique of Common-Emitter Transconductance Stage Using a Low-Frequency-Trap Networkxe2x80x9d, IEEE, Journal of Solid-State Circuits, Vol. 35, No. 8, August 2000, describes the presence of a xe2x80x9ctrapxe2x80x9d filter tuned to the frequency of spacing of the tones used for an intermodulation test. This filter, which is placed in parallel with the biasing circuit, increases the order 3 interception point, but does not change the compression point.
Furthermore, the obligatory construction outside of the integrated circuit of this resistive/inductive/capacitive xe2x80x9ctrapxe2x80x9d filtering network, tuned to frequencies lying between 100 KHz and 10 MHz according to the transmission standards used, represents a major drawback for this technique.
Accordingly, there exists a need for overcoming the disadvantages of the prior art as discussed above.
According to a preferred embodiment of the present invention, a method of biasing a transistor of a radio frequency amplifier stage includes
receiving an input signal at an amplifier transistor, the input signal being provided from a radio frequency signal source;
closed-loop transconductance slaving a time average voltage of a base/emitter or gate/source of the amplifier transistor to a reference voltage corresponding to a desired quiescent current for the amplifier transistor; and
setting an input circuit impedance of a base/emitter or gate/source circuit of the amplifier transistor, as viewed from the base or gate of the amplifier transistor, to a small value at a low frequency of the input signal, and to a large value with respect to an output circuit impedance of the radio frequency signal source at a radio frequency of the input signal.
Further, according to a preferred embodiment of the present invention, an electronic device can provide bias for a transistor of a radio frequency amplifier stage intended for processing a modulated radio frequency signal emanating from a radio frequency source, the electronic device includes:
an input at an amplifier transistor for receiving an input signal provided from a radio frequency signal source; and
closed-loop transconductance slaving means for slaving a time average voltage of the base/emitter or gate/source voltage of the amplifier transistor to a reference voltage corresponding to a desired quiescent current for the amplifier transistor, and wherein an input circuit impedance of a base/emitter or gate/source circuit of the amplifier transistor, as viewed from the base or gate of the amplifier transistor, the input circuit impedance of the base/emitter or gate/source circuit is a small value at a low frequency of the input signal, and large value with respect to an output circuit impedance of the radio frequency signal source at a radio frequency of the input signal.
The invention proposes also a mobile terminal, such as a cellular mobile phone.